MINOR: mux-quic: count in-progress requests

MINOR: mux-quic: count in-progress requests

Add a new qcc member named <nb_hreq>. Its purpose is close to <nb_sc>
which represents the number of attached stream connectors. Both are
incremented inside qc_attach_sc().

The difference is on the decrement operation. While <nb_cs> is
decremented on sedesc detach callback, <nb_hreq> is decremented when the
qcs is locally closed.

In most cases, <nb_hreq> will be decremented before <nb_cs>. However, it
will be the reverse if a stream must be kept alive after detach callback.

The main purpose of this field is to implement http-keep-alive timeout.
Both <nb_sc> and <nb_hreq> must be null to activate the http-keep-alive
timeout.